In recent years, improvements in recording density have been remarkable in the field of magnetic recording media that employ hard disk drives (HDD) and the like. Said recording density has continued to rise at a prodigious rate, increasing approximately 100-fold in the last 10 years.
The technologies that support said improvements in recording density are varied, but one may cite a control technology characterized by sliding between a magnetic head and a magnetic recording medium as one key technology.
As one control technology with said sliding property, there is a technology called the Wintester mode. The Wintester mode uses the CSS (Contact Start Stop) method wherein the basic operations from start-up to stoppage of the magnetic head are contact sliding—flotation—contact sliding relative to the magnetic recording medium, and it has become mainstream in hard disk drives. With this technology, contact sliding of the magnetic head on the magnetic recording medium is unavoidable.
Consequently, with this technology, problems of tribology (an academic domain which studies mechanisms and the like of friction, wear, and lubrication) between the magnetic head and the magnetic recording medium have until now remained a fateful technical challenge.
Efforts to improve the protective film which is laminated onto the magnetic film of the magnetic recording medium are ongoing. The wear resistance and slide resistance of the surface (protective film surface) of this medium are major themes in enhancement of the reliability of the magnetic recording medium.
With respect to said protective film, when the flying height of the magnetic head is reduced in order to seek enhanced recording density of the magnetic recording medium, a high degree of slide durability and excellent planarity are required so as to enable protection of the magnetic recording layer of the magnetic recording medium even when said magnetic recording medium and the magnetic head make incidental contact. Moreover, in order to mitigate the spacing loss of the magnetic recording medium and the magnetic head, it is necessary to thin the thickness of said protective film as much as possible, for example, to a film thickness of 30 Å or less. Thus, with respect to said, protective film, not only is there a strict requirement for smoothness, but also for thinness, fineness, and toughness.
As the material of said protective film, a variety of materials have been proposed, but film composed of carbon (hereinafter “carbon film”) has mainly been adopted from the overall standpoint of film formation properties, durability, and the like. Properties of said carbon film such as hardness, density, and dynamic friction coefficient are clearly reflected in the CSS properties or corrosion resistance properties of the magnetic recording medium.
Said carbon film may be formed by the sputtering method, CVD method, ion-beam evaporation method, or the like. However, with respect to carbon film that is formed by the sputtering method, there is the risk of insufficient durability in cases where, for example, a film thickness of 100 Å or less is produced. With respect to carbon film formed by the CVD method, surface smoothness thereof is low. Therefore, when film thickness is thin, there is the risk that the coverage ratio of the surface of the magnetic recording medium may decrease, and that corrosion of the magnetic recording medium may occur.
On the other hand, compared to carbon film formed by the sputtering method or the CVD method, carbon film that is formed by the ion-beam evaporation method is capable of producing film that has higher hardness and greater smoothness, and that is finer. Patent Document 1 discloses one example of a carbon film forming method by the ion-beam evaporation method.
Patent Document 1 relates to a CVD apparatus and a magnetic-recording-medium manufacturing method, and discloses an ion-beam evaporation method which uses thermal filament and a plasma CVD apparatus.
As described in Patent Document 1, with respect to the ion-beam evaporation method, inside a film formation chamber under a vacuum atmosphere, raw material gas of the hydrocarbon system enters a plasma state by electric discharge between an anode and a filamentous cathode that is thermally energized. The carbon ions and carbon radicals which are generated by excitation and decomposition of said raw material gas accelerate and impact the film formation surface of a substrate that is disposed so as to face said cathode and that has minus potential, with the result that carbon film is stably formed with a high degree of hardness.
As said substrate, a disk-like substrate having a circular aperture at the center is ordinarily used. However, in the case where carbon film is formed on said substrate using the ion-beam evaporation method described in Patent Document 1, the thickness of the portion of carbon film at the edge of said aperture tends to become greater than that of other portions. As a cause of this tendency, it is thought that plasma containing carbon ions is irradiated to cluster in the portion at the edge of said aperture, and thereby raising carbon ion concentration, and also that temperature in the portion at the edge of said aperture is greater than that in other portions, increasing the growth speed of carbon film.
In order to prevent occurrence of thickness irregularities, Patent Document 1 describes a configuration wherein a film thickness correction plate is disposed for the purpose of correcting film thickness on the film formation surface side of the substrate. Specifically, a coin-shaped shield (film formation correction plate) is arranged on the film formation surface side of the aperture of the substrate. By this means, plasma density and carbon ion concentration in the vicinity of the aperture of the substrate are reduced, and the growth speed of carbon film in the portion at the edge of the aperture of the substrate is lowered, promoting planarization and smoothening of the carbon film.
However, planarization and smoothening of the carbon film are insufficient even with disposition of the film formation correction plate described in Patent Document 1. That is, the carbon ions which contribute to formation of carbon film include not only flying components which arrive at the film formation surface of the substrate from a direction that is vertical to the film formation surface of the substrate, but also flying components which arrive at the film formation surface of the substrate by flying from other directions. Consequently, even when said coin-shaped shield is set up, a portion of the flying components, which are the carbon ions and are desired to be intercepted, goes around said coin-shaped shield, and forms carbon film on the substrate, thickening film thickness of the carbon film at the edge of the aperture.
When thickness of the carbon film at the edge portion of the aperture becomes greater than that of other portions, the planarity and smoothness of the carbon film is lost. As a result, the problem arises that it becomes difficult to reduce the distance between the magnetic recording medium and the magnetic head, and that the recording density of the magnetic recording medium cannot be improved.
When the diameter of said coin-shaped shield is enlarged in order to prevent wrap-around of carbon ions, deactivated carbon radicals reach the portion at the edge of the aperture of the substrate, where carbon film of low hardness is formed. Carbon film of low hardness is unable to sufficiently exhibit the functions of a protective film of the magnetic recording medium, and corrosion of the magnetic layer occurs from the edge portion of the aperture, reducing the reliability of the magnetic recording medium.